In some applications, a structural part is exposed to high surface temperatures on a heated surface of the part. An oppositely disposed cooled surface of the part is cooled with a flow of cooling air. The maximum temperature reached by the part is determined by a balance between the amount of heat that enters the part from the heated surface, and the amount of heat removed by the cooling air flow over the cooled surface. Examples of such applications include combustors, nozzles, liners, and turbines in aircraft gas turbine engines.
It has been known to apply a thermal barrier coating system to the heated surface of the part to serve as an insulation that reduces the heat flow into the part and allows it to operate in a hotter external environment. The thermal barrier coating system typically includes a metallic bond coat overlying the metal or ceramic substrate that forms the part, and a ceramic layer overlying the bond coat. The bond coat improves the adherence of the ceramic layer to the substrate. The ceramic layer, which is typically a zirconium-based ceramic, reduces the heat flow into the substrate from the hot surface.
Coated articles having a metal or ceramic substrate, an intermediate thermal barrier coating overlying the substrate, and a low-emissivity metallic top coat over the thermal barrier coating are known. The available thermal barrier coatings typically utilize a ceramic layer overlying a metallic bond coat. The ceramic layer insulates the substrate, and the metallic bond coat improves the adherence of the ceramic layer to the substrate. The low-emissivity top coat reflects some of the thermal energy incident upon the coated article, so that the metal or ceramic part below is exposed to less heat input. While operable, the available systems with metallic top coats have not been practical for use in high-temperature environments such as gas turbines, because the reflective metal degrades and/or volatilizes after only a few minutes of exposure to the high-temperature, corrosive environment.
U.S. Pat. No. 5,851,679 discloses the use of a low-emissivity coating in the form of a ceramic multilayer optical reflector overlying the thermal barrier coating, to further reduce the heat flow into the coated part. This type of low-emissivity top coating is an improvement over metallic low-emissivity coatings, because it is more stable in the gas turbine environment.
However, the inventors have recognized that the available thermal barrier coatings have shortcomings in respect to their performance in conjunction with the ceramic multilayer or metallic low-emissivity top coats. Consequently, there is a need for a thermal barrier coating system which promotes the formation and functioning of low-emissivity top coats. The present invention fulfills this need, and further provides related advantages.